1. Field of the Invention
The present invention relates to the general field of suspension and testing of structures and, more specifically, to a microgravity suspension system used to suspend a test article in a ground test laboratory for the purpose of testing it in a simulated weightless environment.
2. Description of the Related Art
The development and deployment of a new generation of precision spacecraft, satellites, antennas and other similar space structures has prompted increased interest in the establishment of ground testing facilities which include versatile suspension systems to support the structures and allow them to behave as if they were in earth's orbit. The testing of such structures is extremely important before deployment in space since the size and weight of some of these structures can present difficulties in control and movement in space. Also, some of the structures are required to maintain their shape and alignment to within microns. Other structures may be smaller and stiffer, and will be required to slew rapidly, achieving pointing accuracies on the order to microradians, and limit jitter to nanoradians.
Therefore, there is a need to test such structures before deployment to enhance the structure's design or to modify a structure to permit it to achieve its desired performance in space. Such testing can also provide useful data regarding a structure's natural frequencies, damping and modes of vibration which can be critical in the design and manufacture of the structure and future structures.
There have been various approaches that have been taken with ground testing of large structures in order to approximate or nearly approximate the microgravity environment of space. Some systems and methods have been more successful than others in simulating a microgravity environment.
One approach that has been used to allow both horizontal and vertical motion of suspended test structures is through the suspension of the structure with long bungee-type cords which support the structure in a pendulum fashion from a high ceiling. While this particular approach provides somewhat useful data acquisition, the length of the cords involved is inversely proportional to the square of the fundamental frequency of the suspended structure, which may require a large overhead clearance to accommodate the long cords, especially if the fundamental frequency of the structure is below one Hertz. Also, the motion of the suspended structure may be limited to a few inches and the bungee cord itself can contribute unwanted mass, damping and stiffness to the structure, possibly distorting its natural properties.
Another approach at simulating a microgravity environment has utilized zero spring rate mechanisms which are theoretically capable of suspending a structure with an arbitrarily small vertical spring rate, unlike bungee cords where the vertical spring rate is inversely proportional to the length. Suspension damping and mass are also lower than those of bungee cords, although large overhead clearance is still required for horizontal isolation which depends on the pendulum effect. Again, the length of the suspension cables used in this suspension system would be inversely proportional to the square of the structure's fundamental frequency. Also, some zero spring rate mechanisms can have a limited range of motion of just a few inches and, although capable of providing a significant improvement over bungee cords, may still contribute unwanted mass, damping and stiffness to the structure during testing, but to a lesser degree than bungee cords.
A solenoid-type support system which uses electromagnetic force to support the test structure has also been designed and tested. Such a system eliminates friction by avoiding mechanical contact of relatively moving parts. A solenoid-type system can eliminate unwanted mass and stiffness by compensating for it with an active (feedback) control system. Still, however, large overhead clearances may be required for horizontal isolation if the solenoid-type suspension system depends upon on the pendulum effect as well. Horizontal motion of the structure can also be limited to a few inches and the system can have a high electrical power requirement. Also, such a suspension can generate a large amount of heat and a strong magnetic field which may cause distortions in electronically acquired test data.
Another suspension/test system utilizes electropneumatic components which employ an air-lubricated pneumatic cylinder to support the weight of the structure. Structures of various weights are suspended by regulating the air pressure behind the piston in the cylinder. Friction is eliminated by avoiding mechanical contact of relatively moving parts. Suspension stiffness and mass can also be compensated for by a small actively controlled solenoid-type device. However, such a system may still rely upon the pendulum effect for horizontal isolation which may require a large overhead clearance. Motion of the test structure may again be limited to a few inches utilizing such an electropneumatic system.
Thus, there are common problems associated with numerous forms of suspension systems which hinder the effective gathering of data relating to the test structure and which require special overhead clearance which is sometimes prohibitive. Also, since many of these known suspension systems may have a relatively limited range of simulated microgravity motion, it is often difficult to perform the dynamic tests desired for realistic ground testing. Also, deployment and operation of certain space structures, such as space robotic arms, are typically flexible and can undergo large motions, characteristics of which bungee cords and other unidirectional suspension systems are not well suited to handle. As a result, there is a need for an improved microgravity suspension system which eliminates some of the disadvantages which have been associated with prior systems. Such a system would be significantly improved if it allowed for a wider range of motion of the test structure and eliminated the requirements for large overhead clearance.